1. Field of the Invention
This invention relates to an analog video chromakey mixer.
2. Description of Related Art
When still or moving images in digital form are displayed in a computer system, the digital images must generally be decoded and displayed as images on a computer monitor screen. Typically, the monitor screen is the only monitor screen in the computer system. However, those decoded and displayed images must be coordinated with other display signals directed at the same monitor screen, such as those signals directed that the monitor screen by an RGB or VGA monitor driver. Typically, the two sets of signals directed at the monitor screen must be multiplexed in some way.
Generally, it is desired that the two sets of signals must be smoothly multiplexed, with no breaks that would be visible to the human eye. It is also generally desired that the two sets of signals should be multiplexed quickly, so that high quality, high speed images may be displayed. It is also generally desired that any method for multiplexing the two sets of signals should work with a wide variety of computer systems and with a minimum of adaptation required for any of them.
However, one problem that has arisen in the art is that high quality, high speed multiplexing of analog and digital video signals can be difficult. For example, if it were desired to digitize the analog video signals and multiplex them with the digital signals entirely digitally, it could require an A/D converter that produced 16 million colors (24 bits) at a 75 MHz pixel rate. Present A/D converters do not operate at this combination of precision and speed, at least not at anything near a reasonable cost for a personal computer system.
One method of the prior art has been to multiplex the digital data provided by the computer system""s processor (or CPU) to the monitor driver. While this method sometimes achieves the goal of synchronizing digital and analog video sources, it has the drawback that it requires substantial information about the method of color encoding used by the VGA monitor driver. As monitor drivers have been changed with improvements in monitors and in drivers, this method also has the drawback that it may fail to work for certain classes of monitor drivers.
Accordingly, it is an object of the invention to provide an improved technique for mixing picture signals directed at a monitor screen.
The invention provides an improved technique for mixing picture signals directed at a monitor screen.
In a preferred embodiment, two analog video signals, such as an analog VGA input and an analog RGB signal produced in response to a stored digital still or moving image, may be multiplexed in analog form. An analog chromakey mixer detects a background color in the first video signal and replaces the portion of that first video signal with the second video signal.
In a preferred embodiment, the time delays of the first video signal and the second video signal may be adjusted so that they reach the monitor screen (by means of a multiplexer output) at the same time. An alignment detector may attempt to align two known signals (such as a VGA sync signal and a signal generated for this purpose), and may adjust a set of time delays in the analog chromakey mixer until the time difference between the first and second video signals falls below a threshold.